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Sat Mar 2, 2019, 04:51 AM

On the combustion of biomass in oxygen enriched carbon dioxide atmospheres.

Last edited Sat Mar 2, 2019, 08:54 AM - Edit history (2)

The paper I'll discuss in this thread is this one: Combustion Characteristics and Pollutant Emissions in Transient Oxy-Combustion of a Single Biomass Particle: A Numerical Study (Wang et al Energy Fuels, 2019, 33 (2), pp 1556–1569)

In general, I'm an opponent of so called "renewable energy" since I think the very term, owing to the low energy to mass ratio associated with it which has huge environmental implications, as well the fact that they intermittent, which impose a high thermodynamic (and thus, in another way, environmental) cost, represents an absurd, if hidden, oxymoron. "Renewable energy" is not really "renewable." It's consumptive.

These limitations are the reason that solar and wind for example, are useless to address climate change and is the reason why, after spending trillions of dollars on them, they have done nothing at all to slow the acceleration of climate degradation via the destruction of the planetary atmosphere. My view if they were not trivial forms of energy - although it is unlikely that they will ever be anything other than trivial - their environmental consequences would be obvious, but they are not obvious, layered under so much popular hype, obfuscation and hand waving, although it one actually looks one can in fact find out what those environmental costs actually are.

Actually the most successful form of so called "renewable energy" is also the most deadly: The combustion of biomass is responsible for slightly less than one half of the 7 million air pollution deaths that occur each year.

Global, regional, and national comparative risk assessment of 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks, 1990–2015: a systematic analysis for the Global Burden of Disease Study 2015 (Lancet 2016; 388: 1659–724)

However the combustion of biomass is potentially capable of becoming a very clean form of energy, and to the extent that the carbon dioxide can be captured and put to use, it is technically feasible that it could actually be carbon negative, although the claims that it is carbon neutral as currently practiced is at best, dubious.

The means to doing this would involve combustion in a closed system, that is a system that has no smokestack, no exhaust. This is only possible really under two conditions, one being the famous and often discussed "chemical looping" process where an oxygen carrier, generally a multivalent metal such as iron or cerium, is oxidized by air and then reduced by biomass by what is effectively combustion, releasing energy. I like to read about these systems for fun, but my feeling is that in practical engineering terms there are certain mass transfer features that make them problematic. To my knowledge, no large scale or even pilot scale chemical looping device exists. The second condition is to burn the fuel in pure (or as we shall see) oxygen mixtures other than air.

These conditions appeal to me, as I have been very interested in thermochemical water and carbon dioxide splitting cycles and have written in this space (and elsewhere) about them. Both types of cycles are designed to produce pure oxygen; and there are also cycles - albeit somewhat more obscure - that produce hydrogen (or its potential surrogate, carbon monoxide) and equimolar mixtures of carbon dioxide and oxygen.

I've been fascinated by this latter stream, equimolar oxygen and carbon dioxide mixtures, because I imagine many useful applications for them, but I haven't seen very much written about them, at least until I came across the recent paper cited at the outset of this post.

Of course, simply because I haven't heard of something about which I've speculated doesn't imply that it hasn't already been studied in significant detail; I'm not that smart nor am I that well read. This paper refers to actual experiments that have been done along these lines. Here is reference 37 in the paper, which I have not read but will access in the future:

(37) Khatami, R.; Stivers, C.; Joshi, K.; Levendis, Y. A.; Sarofim, A. F. Combustion behaviour of single particles from three different coal ranks and from sugar cane bagasse in O2/N2 and O2/CO2 atmospheres. Combust. Flame 2012, 159, 1253−1271.

The paper currently under discussion is a paper about the mathematical modeling the combustion of biomass in atmospheres other than air, and it compares the mathematical modeling therein with the results reported in reference 37.

From the introductory paragraphs of the paper:

The growing concerns about global warming and issues around energy security have turned renewable sources of energy into the main means of addressing world energy demands.1 Biomass is regarded as a promising renewable fuel and has seen an increased tendency in use. Pulverized combustion for power generation, similar to that for coal, is perhaps the most common technology for utilizing biomass energy,2 which is being promoted worldwide.3 A large amount of carbon dioxide (CO2) generated from coal-fired power plants is now a serious issue, and thus, different methods have been developed for carbon capture and storage (CCS).4 Among these, oxy-fuel combustion is regarded as the most promising CCS technique for power station utilization.4 It is, however, noted that provision of oxygen through low-carbon processes is an important prerequisite to this. Due to carbon neutrality of biomass, application of CCS to biomass-fired stations can lead to negative carbon generation, which is an attractive method of decarbonising the atmosphere. Successful implementation of oxy-combustion of biomass requires an understanding of the underlying physicochemical processes under O2/CO2 by O2/ N2 atmospheres. Yet, some aspects of oxy-coal/biomass combustion including the volatiles matter evolution, homogeneous reactions, and heterogeneous combustion of char are quite complex and far from being fully understood and thus require further research...

What follows is a few paragraphs which represent a brief review of several papers on combustion in non-air oxygen enriched atmospheres as well as a brief reference to a problem with biomass combustion, corrosion of the combustion chambers owing to the serious (and often deadly) pollutants it generates, specifically nitrogen oxides and sulfates. Then the raison d’κtre for the paper is given:

...The preceding review of the literature indicates that, so far, most investigations have been focused on coal or char combustion, and there are only a few studies on a single biomass particle under oxy-fuel conditions. More importantly, the existence of inconsistent and sometimes conflicting results on NOx and SOx emissions highly necessitates conduction of further investigations. Thus, the current work performs a numerical study of combustion of a single biomass particle under O2/N2 and O2/CO2 environments with varying oxygen concentration. The spatiotemporal distributions of the temperature and species fields are analyzed, and NOx and SOx emissions are evaluated to provide a deeper insight into the underlying physicochemical phenomena.

A little bit about the theory behind the model which involves the numerical evaluation of a bunch of differential equations.

Some flavor:

The numerical simulations are conducted by using ANSYS Fluent 15.0. A Euler−Lagrange numerical model with standard k−ε turbulence model, weighted-sum-of-gray-gases model (WSGGM), and P-1 radiation model (spherical harmonic method) was implemented.38 Further, the SIMPLE algorithm was used for velocity−pressure coupling,39 and the effect of gravity was added to the numerical simulations. The computational model simultaneously solves the following governing equations. The conservation of mass is given by

Conservation of momentum in axial and radial directions read

The balance of energy for the reactive flow is written as

and the conservation of species (sic) is expressed by

The ideal gas law for the multicomponent gas is written as

There are, of course, many far more sophisticated mathematical models for multi-component gas mixtures built around various gas equations, but as one can glean, there is already a lot of computer time here invested in this project, and when one gets to the meat of the results, they're fairly accurate when compared with the experimental results found in reference 37, generally under 3.00%, with the exception that is represented by the less than interesting case of depleted air, 20% oxygen and 80% nitrogen.

Here's a picture of the geometry of the simulating chamber:

The caption:

Figure 1. Schematic of axis-symmetric domain used for the numerical

A graphic on nitrogen flows, including some hydrogen cyanide:

The caption:

Figure 2. Fuel−NOx pathways

Here is the flavor of what the simulations graphic output looks like, this one referring to the temperature of a biomass particle falling in the chambers of the experimental system being modeled.

The caption:

Figure 4. Spatiotemporal distribution of the mass fraction of CO2: (a) 37% O2/CO2 (2, 6, 10, 14, and 18 ms) and (b) 100% O2 (3, 5, 7, 9, and 11 ms).

Another interesting example of the same:

The caption:

Figure 8. History of mass-averaged mole fraction of the major gaseous species during single biomass particle combustion: (a) 27% O2 and 71% N2, (b) 100% O2, (c) 37% O2 and 63% CO2, and (d) 77% O2 and 23% CO2.

Another interesting one:

The caption:

Figure 9. Overall PPM in different atmospheres during single-Bagasse particle combustion: (a) NO and (b) SO2

A nice graphical overview of all the results of these simulations:

The caption:

Figure 11. Species versus particle mass reduction during single-Bagasse particle combustion: (a and b) 37% O2 and 63% N2, (c and d) 77% O2 and 23% N2, (e and f) 37% O2 and 63% CO2, and (g and h) 77% O2 and 23% CO2.

Another nice summary graphic summary:

The caption:

Figure 12. Species formation percentage for devolatilization and char combustion processes under different gas conditions: (a) HCN, (b) NH3, (c) SO2, and (d) H2S.

A few remarks out of the conclusion that are of interest:

• The combustion behavior of single biomass particle is significantly different in O2/N2 and O2/CO2 atmospheres. The volatile matters combust prior to ignition of the particle in O2/CO2, while the volatiles and chars combust sequentially in O2/N2 conditions.

• Under CO2 atmosphere, the production and depletion process of CO is majorly affected by the large amount of CO2 existing in the background gas.

Tomorrow morning I'll be attending a lecture of how we might "adapt" to climate change in New Jersey.



The fact is we will be forced to adapt, as best we can...or die.

The reason is that we are doing nothing serious to address climate change other than to dump responsibility for our indifference on all future generations of all living things.

I'm sorry, but solar roofs on McMansions, and converting the entire continental shelf into industrial parks for wind turbines that will be in landfills in twenty years won't work, nor will worshiping Elon Musk's stupid car for rich people, nor any of the other horseshit we hear about endlessly while things deteriorate faster and faster.

None of this has worked; none of it is working, and again, and again and again, it won't work.

Sorry, it's just reality.

It does seem that it's technically feasible to find away out, but we'd rather recite dogma than actually try something different.

However that is, little obscure papers like this, are a little bit of hope, and as I near the end of my life it's all I have...a little bit of hope.

Have a pleasant weekend.

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